1. Field of the Invention
The present invention relates to an ultrasonic sensor transmitting and/or receiving ultrasonic waves having frequencies adjusted according to ambient temperature.
2. Description of Related Art
An example of an ultrasonic sensor is disclosed in JP-A-8-237796. The ultrasonic sensor includes a piezoelectric vibrator disposed in a cylindrical casing in contact with a thin end wall that closes one end of the cylindrical casing. In transmitting operation, the thin end wall vibrated by the piezoelectric vibrator transmits ultrasonic waves. In receiving operation, ultrasonic waves received by the thin end wall are transmitted to the piezoelectric vibrator and converted into electrical signals.
In order to obtain high efficiency, the thin end plate is vibrated at its resonant frequency by the piezoelectric vibrator. However, since a Young's modulus of the thin end wall changes according to ambient temperature, the resonant frequency also changes according to the ambient temperature. Therefore, there is a problem that the transmitting or receiving efficiency of the ultrasonic sensor decreases as the ambient temperature deviates from a standard temperature.